Conventionally, hydrogen has been stored in a pressure-resistant vessel under a high pressure or stored as a cryogenic liquid, being cooled to an extremely low temperature. However, said storage methods have revealed some drawbacks that: transfer is very difficult, since the hydrogen is stored in a large-sized vessel; amount of hydrogen stored in a vessel is limited, due to low density of hydrogen; and, heat is required to release hydrogen to the outside.
On the other hand, several metals and alloys have been known to permit reversible storage and release of hydrogen. In this regard, they have been considered as a superior hydrogen-storage material, due to their high hydrogen-storage efficiency. Solid-phase metal or alloy system can store large amounts of hydrogen by absorbing hydrogen with a high density and by forming a metal hydride under a specific temperature and pressure or an electrochemical conditions, and hydrogen can be released by changing the said conditions. The metal hydride has an advantage of high-density hydrogen-storage for a long time, since it is formed by the intrusion of hydrogen atoms to the crystal lattice of a metal.
In general, hydrogen-storage alloy system must have a hydrogenation characteristics such as a proper plateau pressure, high hydrogen-storage capacity and hydriding rate, and long cycle-life. A variety of metallic materials for hydrogen-storage to meet the said requirements have been proposed in the art, e.g., Mg, Mg-Ni, Mg-Cu, Ti-Fe, Ti-Ni, R-Ni and R-Co alloy systems(wherein, R is a rare-earth metal).
Of these metals, Mg alloy system, though it can store a relatively large amounts of hydrogen per unit weight of the storage material, heat energy should be supplied to release hydrogen stored in the alloy, owing to its low hydrogen dissociation equilibrium pressure at room temperature. Moreover, release of hydrogen can be made over a long time, only under a high temperature of over 250.degree. C. along with the consumption of large amounts of energy.
On the other hand, rare-earth metal alloy, has demerits that hydrogen-storage capacity per unit weight is lower than any other hydrogen-storage material and it is very expensive, though it can efficiently absorb and release hydrogen at room temperature, grounded that it has the hydrogen dissociation equilibrium pressure in the order of several atmospheric pressures at room temperature.
Ti-Fe alloy system which has been considered as a typical and superior material of the titanium alloy systems, has the advantages that it is relatively inexpensive and the hydrogen dissociation equilibrium pressure of hydrogen is several atmospheric pressure at room temperature. However, since it requires a high temperature of about 350.degree. C. and a high pressure of over 30 atms for initial hydrogenation, the alloy system provides relatively low hydrogen absorption/desorption rate and it has a hysteresis thereof, finally to hinder the complete release of hydrogen stored therein.
Under the circumstances, a variety of approaches have been made to solve the problems of the prior art and to develop an improved material which has a high hydrogen-storage efficiency, a proper hydrogen dissociation equilibrium pressure and a high absorption/desorption rate.
In this regard, Ti-Mn alloy system has been reported to have a high hydrogen-storage efficiency and a proper hydrogen dissociation equilibrium pressure, since it has a high affinity for hydrogen and low atomic weight to allow large amounts of hydrogen-storage per unit weight.
In specific, Ti.sub.0.9 Zr.sub.0.1 Mn.sub.1.4 V.sub.0.2 Cr.sub.0.4 alloy system which has a hydrogen-storage capacity of 1.94 wt % and a dissociation pressure of 8 to 10 atm at 20.degree. C., has been suggested as a superior hydrogen-storage alloy(see: T. Gamo et al, Int. J. Hydrogen Energy, 10(1):39-47(1985)). However, said alloy system does not reach a stage of practical application in the art, since it has still low hydrogen-storage capacity.